In oil and gas wells, it is important to be able to monitor and detect corrosion and related damage to pipes (e.g., casings, tubing, etc.) in the wells. Downhole or in-situ pipe inspection typically involve electromagnetic (EM) inspection tools that are conveyed through the interior of the pipe by wireline, slickline, coiled tubing and similar conveyances. The inspection tools typically induce a voltage on the pipes and log or record the voltage along the length of the pipes to determine pipe thickness and other pipe characteristics. These pipe characteristics may then be used to identify the location of corrosion, metal loss, and other defects along the pipes.
There are generally two categories of EM inspection tools: magnetic flux leakage (MFL) based tools, and eddy current (EC) based tools. In general, inspection tools that are based on magnetic flux leakage are considered more suitable for inspecting a single pipe at a time, while those that are based on eddy current are considered better for simultaneous inspection of multiple pipes.
Eddy current-based tools may further be divided into two categories: frequency-domain tools, and time-domain tools. Frequency-domain eddy current-based tools have a transmitter typically in the form of a coil that is excited by a continuous sinusoidal signal to generate a transient magnetic field in the vicinity of the pipes. This transient magnetic field, called a primary field, induces eddy currents in the pipes that in turn generate secondary magnetic fields. The primary and secondary magnetic fields are then detected by a receiver, also typically in the form of a coil, that measures the voltages induced in the receiver by the magnetic fields. The receiver may be the same as the transmitter or it may be a separate receiver that may be co-located with the transmitter or placed further away on the tool from the transmitter. Pipe thickness and other pipe characteristics may then be determined by processing the induced voltages measured by the receiver using well-known inversion techniques.
In time-domain eddy current-based tools, also called pulsed EC (PEC) tools, the transmitter is excited by a pulse signal. As with frequency-domain tools, the pulse signal generates a transient magnetic field as it transitions from an OFF state to an ON state (or from ON to OFF state). The primary field induces eddy currents in the pipes that in turn generate secondary magnetic fields. The primary and secondary fields induce voltages that are then measured by the receiver and processed to determine pipe thickness and other characteristics of the pipes.
Pipes can sometimes have residual magnetism (i.e., be magnetized) because of prior exposure to magnetic fields. The residual magnetism results in a static and non-uniform residual magnetic field about the pipes that can affect the accuracy of eddy current-based tools. Specifically, the residual magnetic field can induce an undesired residual voltage in the receiver as the tool passes through the pipes. The undesired residual voltage may add to or subtract from the voltage induced by the primary and secondary fields in the receiver, resulting in incorrect receiver measurements. The size of the undesired residual voltage may depend on several factors, including the strength of the residual magnetic field, the velocity at which the coil moves through the field, and the dimensions and number of windings in the receiver. If not properly taken into account, these undesired residual voltages can cause eddy current-based tools to produce spurious results.
Accordingly, a need exists for a way to minimize or eliminate the effect of residual magnetism in eddy current-based inspection tools.